1  /* Default mutex implementation struct definitions.
       2     Copyright (C) 2019-2023 Free Software Foundation, Inc.
       3     This file is part of the GNU C Library.
       4  
       5     The GNU C Library is free software; you can redistribute it and/or
       6     modify it under the terms of the GNU Lesser General Public
       7     License as published by the Free Software Foundation; either
       8     version 2.1 of the License, or (at your option) any later version.
       9  
      10     The GNU C Library is distributed in the hope that it will be useful,
      11     but WITHOUT ANY WARRANTY; without even the implied warranty of
      12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      13     Lesser General Public License for more details.
      14  
      15     You should have received a copy of the GNU Lesser General Public
      16     License along with the GNU C Library; if not, see
      17     <http://www.gnu.org/licenses/>.  */
      18  
      19  #ifndef _THREAD_MUTEX_INTERNAL_H
      20  #define _THREAD_MUTEX_INTERNAL_H 1
      21  
      22  /* Generic struct for both POSIX and C11 mutexes.  New ports are expected
      23     to use the default layout, however architecture can redefine it to
      24     add arch-specific extension (such as lock-elision).  The struct have
      25     a size of 32 bytes on LP32 and 40 bytes on LP64 architectures.  */
      26  
      27  struct __pthread_mutex_s
      28  {
      29    int __lock __LOCK_ALIGNMENT;
      30    unsigned int __count;
      31    int __owner;
      32  #if __WORDSIZE == 64
      33    unsigned int __nusers;
      34  #endif
      35    /* KIND must stay at this position in the structure to maintain
      36       binary compatibility with static initializers.
      37  
      38       Concurrency notes:
      39       The __kind of a mutex is initialized either by the static
      40       PTHREAD_MUTEX_INITIALIZER or by a call to pthread_mutex_init.
      41  
      42       After a mutex has been initialized, the __kind of a mutex is usually not
      43       changed.  BUT it can be set to -1 in pthread_mutex_destroy or elision can
      44       be enabled.  This is done concurrently in the pthread_mutex_*lock
      45       functions by using the macro FORCE_ELISION. This macro is only defined
      46       for architectures which supports lock elision.
      47  
      48       For elision, there are the flags PTHREAD_MUTEX_ELISION_NP and
      49       PTHREAD_MUTEX_NO_ELISION_NP which can be set in addition to the already
      50       set type of a mutex.  Before a mutex is initialized, only
      51       PTHREAD_MUTEX_NO_ELISION_NP can be set with pthread_mutexattr_settype.
      52  
      53       After a mutex has been initialized, the functions pthread_mutex_*lock can
      54       enable elision - if the mutex-type and the machine supports it - by
      55       setting the flag PTHREAD_MUTEX_ELISION_NP. This is done concurrently.
      56       Afterwards the lock / unlock functions are using specific elision
      57       code-paths.  */
      58    int __kind;
      59  #if __WORDSIZE != 64
      60    unsigned int __nusers;
      61  #endif
      62  #if __WORDSIZE == 64
      63    int __spins;
      64    __pthread_list_t __list;
      65  # define __PTHREAD_MUTEX_HAVE_PREV      1
      66  #else
      67    __extension__ union
      68    {
      69      int __spins;
      70      __pthread_slist_t __list;
      71    };
      72  # define __PTHREAD_MUTEX_HAVE_PREV      0
      73  #endif
      74  };
      75  
      76  #if __PTHREAD_MUTEX_HAVE_PREV == 1
      77  # define __PTHREAD_MUTEX_INITIALIZER(__kind) \
      78    0, 0, 0, 0, __kind, 0, { 0, 0 }
      79  #else
      80  # define __PTHREAD_MUTEX_INITIALIZER(__kind) \
      81    0, 0, 0, __kind, 0, { 0 }
      82  #endif
      83  
      84  #endif